Detection of a Metal or a Magnetic Object

ABSTRACT

A measuring apparatus for detecting a metal object includes two emission coils, a magnetoresistive measuring device, and a control device. The emission coils are configured to produce superimposed magnetic fields. The magnetoresistive measuring device is in the region of both magnetic fields, and is configured to emit an output signal which is dependent on the magnetic field. The control device is configured to supply the emission coils with alternating voltages such that the value of the alternating voltage component of the output signal, which is time synchronized with the alternating voltages, is minimized. The control device is further configured to detect the object when the ratio of the alternating voltages does not correspond to the distances between the magnetoresistive measuring device and the emission coils.

When performing certain types of work on workpieces, there is a risk that an object concealed in the workpiece may be damaged by the work. For example, when drilling into a wall, a water pipe, electrical cable, or gas line running inside the wall may be damaged. On the other hand, it may be desirable to perform the work in such a precise manner that work is also performed on an object concealed in the workpiece, for example, if the hole from the above example is to run through an iron reinforcement or a supporting structure inside the wall.

BACKGROUND OF THE INVENTION

Coil-based metal detectors for detecting such a concealed object are known in the art. Such detectors generate a magnetic field in a measurement region. If a metallic object is in the measurement region, the object is detected because of its influence on the generated magnetic field. To determine the generated magnetic field, at least two receiving coils are often used, which are oriented and connected to one another in such a way that the measurement signal provided jointly by both receiving coils approaches zero in the absence of a metallic object in the measurement region (differential measurement). In one variant, a plurality of transmitting coils is used to generate the magnetic field, the coils being activated in such a way that the measured signal in the two receiving coils approaches zero, independently of the presence of a metallic object in the measurement region (field-compensated measurement).

DE 10 2007 053 881 A1 discloses a measurement method for determining the position or the angle of a coil with respect to two other coils. In order to do this, an alternating magnetic field is generated by means of two transmitting coils arranged at an angle to one another. A receiving coil is brought into the alternating magnetic field and the activation of the transmitting coils is changed such that the same voltage is induced in the receiving coil by each of the transmitting coils. A ratio of current values supplied to the transmitting coils provides a measure of a determination of a position and/or angle of the receiving coil with respect to the transmitting coils.

DE 10 2004 047 189 A1 discloses a metal detector having printed coils.

The object of the invention is to provide a simple and accurate detector for a metallic object. An additional object of the invention is to specify a method for determining the metallic object.

DISCLOSURE OF THE INVENTION

The invention achieves these objects by means of a measuring apparatus having the features of claim 1 and a method having the features of claim 10. Dependent claims provide preferred embodiments.

A measuring apparatus for detecting a metallic object comprises two transmitting coils for generating superimposed magnetic fields, a magnetoresistive measuring device, in particular, a measuring device having Hall sensors, in the region of the two magnetic fields for providing an output signal dependent on the magnetic field, and a control device for supplying the transmitting coils with alternating voltages in such a way that the magnitude of an AC voltage component, which is synchronous with the alternating voltages, of the output signal of the measuring device is minimized. The control device is adapted to detect the object if the ratio of the alternating voltages does not correspond to the ratio of the distances of the measuring device from the transmitting coils.

The measuring apparatus can perform a field-compensated and differential measurement and thereby provide an exact measurement result that is resistant to interference. In addition, magnetoresistive measuring devices can be used, which are substantially smaller than conventional coils for determining magnetic fields. This makes possible a highly compact construction of the measuring apparatus and highly integrated measuring arrangements in close physical proximity. Unlike coils, magnetoresistive sensors measure the magnetic field and not the time-based change in the magnetic flux. When generating alternating fields by means of square-wave signals, this has the advantage of allowing the influence of the object to be measured over the entire duration of the half-cycle of the square-wave excitation, instead of only over the short period of field change in the slope region. In this way, it is possible to increase measurement accuracy.

The alternating voltages are preferably AC voltages that are phase-shifted to one another, preferably phase-shifted by 180°, in order to change the magnitude and phase of the magnetic fields of the transmitting coils periodically. The AC voltages enable synchronous demodulation, which makes it possible to suppress interfering signals having frequencies unequal to the modulation frequency in a highly effective manner. In addition, it is possible to generate alternating magnetic fields via the AC voltages in order to induce eddy currents in non-magnetic materials such as copper, with which they can then be detected.

In a first variant, the measuring device can comprise a plurality of sensors spaced apart from one another for magnetic field determination, with the sensors being aligned with one another and connected to one another in such a way that output signals from the sensors add up to zero when the magnetic fields at the sensors are equal, and main field directions of the transmitting coils and preferred directions of the sensors are parallel to one another. For example, Hall sensors oriented antiparallel can be used in a series connection in order to determine a resulting magnetic field in the region of the two transmitting coils economically and precisely.

The sensors can have preferred directions that run parallel to one another, and the signals from the sensors can be subtracted from one another. Alternatively to this, the preferred directions of the sensors can be aligned antiparallel, and the signals from the sensors can be added to one another. A differential amplifier can be used for addition or subtraction, or the sensors can be correspondingly connected to one another.

In principle, any kind of sensor that determines a magnetic field is suitable. Such sensors can have small dimensions so that the measuring device can be miniaturized. A spatial resolution can thus be increased in an embodiment up to a graphically representable range.

The transmitting coils lie advantageously on top of each other in layers parallel to one another, thus facilitating a matrix-like arrangement of a plurality of transmitting coils for one or a plurality of measuring devices. The transmitting coils can be air coils, in particular printed circuits (“printed coils”) formed on a printed circuit board, so that manufacturing can be of low complexity and therefore inexpensive.

One of the sensors can be surrounded by one of the transmitting coils and another of the sensors can lie outside the transmitting coil. By choosing the specific positions of the sensors, two or more sensors can be used in order to provide a signal which is in total proportional to the resulting magnetic field, and which relates to a plurality of points in the region of the transmitting coils.

In a second variant, the measuring device can comprise a sensor for determining a magnetic field gradient, with main field directions of the transmitting coils running parallel to each other and a preferred direction of the sensor running perpendicular or parallel to the main field directions. High-precision sensors for magnetic field gradients are available, for example, as AMR (anisotropic magnetoresistive effect), GMR (giant magnetoresistive effect), CMR (colossal magnetoresistive effect), TMR (tunnel magnetoresistance), or planar Hall sensors. Such sensors can also be obtained inexpensively as standard components. In another embodiment, a sensor not based on the magnetoresistive effect, for example a SQUID sensor, can also be used.

The transmitting coils can be arranged essentially next to one another in a layer, with the preferred direction of the sensor running parallel or perpendicular to this layer. As a result, a measuring arrangement can be accommodated on a printed circuit board that is populated only on one side, which can lower a unit price of the measuring apparatus.

The transmitting coils can essentially be D-shaped, with the backs of the D-shapes facing one another and the sensor being arranged between the backs of the D-shapes. In this way, it is possible to achieve a very compact construction in connection with a sensor for magnetic field gradients.

The sensor is preferably arranged essentially in the layer of the transmitting coils, and another sensor is provided in a layer parallel to this layer, with preferred directions of the sensor and the other sensor being perpendicular to one another. As a result, the accuracy and universality of the measuring apparatus can be increased.

Furthermore, the invention comprises a measuring method for detecting a metallic object comprising steps of supplying two transmitting coils with alternating voltages in order to generate superimposed magnetic fields, of determining an output signal, which is dependent on the magnetic field, of a magnetoresistive measuring device in the region of the two magnetic fields, with the supply of the transmitting coils with alternating voltages taking place in such a way that the magnitude of an AC voltage component, which is synchronous with the alternating voltages, of the output signal of the measuring device is minimized, and of detecting the object if the ratio of the alternating voltages does not correspond to a ratio of the distances of the measuring device from the transmitting coils.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is described in greater detail below with respect to the included drawings, where:

FIG. 1 shows a block diagram of a measuring apparatus;

FIG. 2 shows an arrangement of magnetoresistive measuring devices for the measuring apparatus of FIG. 1;

FIG. 3 shows arrangements of a plurality of transmitting coils on the measuring apparatus of FIG. 1;

FIG. 4 shows arrangements of magnetic field sensors and transmitting coils for the measuring apparatus of FIG. 1;

FIG. 5 shows arrangements of magnetic field gradient sensors and transmitting coils for the measuring device of FIG. 1; and

FIG. 6 shows a flow diagram for a method for detecting a metallic object using the measuring apparatus of FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a block diagram of a measuring apparatus 100. The measuring apparatus 100 is part of a metal detector 105 for detecting metallic objects made, for example, of material containing iron.

A clock generator 110 has two outputs at which it provides phase-shifted periodic alternating signals, preferably phase-shifted by 180°. The alternating signals can in particular comprise square-wave, triangular, or sinusoidal signals. The outputs of the clock generator are connected to a first controllable amplifier 115 and a second controllable amplifier 120. Each of the controllable amplifiers 115, 120 has a control input via which it receives a signal, which controls a gain of the controllable amplifier 115, 120. An output of the first controllable amplifier 115 is connected to a first transmitting coil 125 and an output of the second controllable amplifier 120 is connected to a second transmitting coil 130. Remaining ends of the transmitting coils 125 and 130 are respectively electrically connected to a defined potential.

As indicated by the dots at the transmitting coils 125 and 130, the transmitting coils 125 and 130 are oriented in opposite directions. When supplied with opposite voltages with respect to the defined potential, the transmitting coils 125, 130 establish magnetic fields having the same orientations.

The same effect can also be achieved by supplying rectified voltage having alternately varying amplitude with respect to the defined potential; in this case, currents having a superimposed DC component flow. In this case, the orientation of the transmitting coil in the same direction as well as in the opposite direction makes sense. In all cases, the magnetic field sensors are to be aligned with one another and connected in such a way that there is a constant signal at the output of the magnetoresistive measuring apparatus 135 in the object-free case. This signal component corresponds to the superimposed DC component of the currents. In order not to control the input amplifier 140 unnecessarily, the input amplifier can be DC-decoupled using a capacitor.

A magnetoresistive measuring device 135 is connected to an input amplifier 140. The input amplifier 140 is shown with a constant gain; however, in other embodiments, a gain of the input amplifier 140 can also be controllable. As a result, for example, a spatial resolution and/or sensitivity of the measuring apparatus 100 can be capable of being influenced and controlled, for example, based on a measured value.

The output of the input amplifier 140 is connected to a synchronous demodulator 145. The synchronous demodulator 145 is furthermore connected to the clock generator 110, from which it receives a clock signal that indicates the phase angle of the signals provided at the outputs of the clock generator 110. In a simple embodiment in which the signals provided by the clock generator 110 are symmetrical square-wave signals, one of the output signals can be used as a clock signal. The synchronous demodulator 145 essentially interconnects the measurement signal received from the input amplifier 140 alternately at its upper and lower output based on the clock signal provided by the clock generator 110.

The two outputs of the synchronous demodulator 145 are connected to an integrator (integrating comparator) 150, which is shown here as an operational amplifier connected to two resistors and two capacitors. Other embodiments are also possible, for example as an active low pass filter. A digital embodiment following the synchronous demodulator is also conceivable, in which the signal at the outputs of the synchronous demodulator is converted from analog to digital at one or a plurality of instants within a half-cycle and then compared to the corresponding value from the next half-cycle. The difference is integrated and, for example, reconverted to an analog signal and used to control the amplifiers. Whereas the synchronous demodulator 145 provides the measurement signal received from the input amplifier 140 at its lower output, the integrator 150 integrates this signal over time and provides the result at its output. Whereas the synchronous demodulator 145 provides the measurement signal received from the input amplifier 140 at its upper output, this signal is inverted and integrated over time by the integrator 150, and the result is provided at the output of the integrator 150. The voltage at the output of the integrator 150 is the integral of the difference of the low pass-filtered outputs of the synchronous demodulator 145.

If the superimposed magnetic field of the transmitting coils 125 and 130 is equal in magnitude and direction at the magnetoresistive measuring device 135, then the signals provided at the outputs of the synchronous demodulator 145 are on average equal over time, and a signal that approaches zero (ground) is provided at the output of the integrator 150. However, if the influence of the magnetic field of one of the transmitting coils 125, 130 predominates, then the signals provided at the outputs of the synchronous demodulator 145 are on average no longer equal, and a positive or negative signal is provided at the output of the integrator 150.

The signal provided by the integrator 150 is provided for further processing via a connector 155. In addition, a microcomputer 175 is connected to the control inputs of the controllable amplifiers 115, 120. The microcomputer 175 compares the provided signal to a threshold value and outputs a signal at an output 180, which indicates the metallic object. The signal can be presented to a user of the metal detector 105 optically and/or acoustically.

Furthermore, the microcomputer 175 can perform additional processing of the signals tapped from the control inputs of the controllable amplifiers 115, 120, and can control parameters of the measuring apparatus 100 based on the signals. For example, a frequency or signal shape of the alternating voltages at the outputs of the clock generator 110 can be varied, or a sensitivity of the receiving amplifier 140 can be changed. In another embodiment, more of the displayed elements of the measuring apparatus 100 are implemented by the microcomputer 175, for example the clock generator 110, the synchronous demodulator 145, or the integrator 150.

The same signal from the integrator 150 is also used to control the gains of the controllable amplifiers 115 and 120, with the second controllable amplifier 120 being directly connected to the output of the integrator 150, and the first controllable amplifier 115 being connected to the output of the integrator 150 by means of an inverter 160. The inverter 160 causes an inversion of the signal provided to it in such a way that the gain of the first controllable amplifier 115 increases depending on the output signal of the integrator 150 to the degree that the gain of the second controllable amplifier 120 decreases, and vice versa. It is also conceivable that only the gain of one of the controllable amplifiers is controlled, while the gain of the second controllable amplifier is kept at a fixed value.

A metallic object 170 is depicted in the region of the transmitting coils 125, 130. The metallic object 170 is at different distances from the transmitting coils 125 and 130 and therefore has a different degree of influence on magnetic fields of the transmitting coils 125, 130 having equal strength. The measuring apparatus 100 is adapted to balance out this influence via an opposite change in the gains of the controllable amplifiers 115, 120. If only one of the two controllable amplifiers 115, 120 is controlled and the other is fixed, then balancing out is not achieved by means of an opposite change, but rather only with a directed change.

FIG. 2 shows an arrangement 200 of two magnetoresistive magnetic field sensors for use with the measuring device of FIG. 1. A first Hall sensor 210 and a second Hall sensor 220 have opposite preferred directions and are connected in series in such a way that they provide a signal that totals zero in a homogeneous magnetic field. The Hall sensors 210, 220 are aligned with their preferred directions parallel (210) and antiparallel (220) to the main field direction of the magnetic field generated in the object-free case by the transmitting coils. The signal from the Hall sensors 210, 220 is routed to the input amplifier 140 in FIG. 1.

The Hall sensors 210 and 220 are necessarily at a certain distance from one another so that they measure magnetic fields at different locations. An output signal from the sensors 210, 220 thus results if the sensors 210, 220 are exposed in their preferred directions to magnetic fields of varying strengths. For example, this is the case if the magnetic field of one of the transmitting coils 125, 130 of FIG. 1 is more strongly influenced by a metallic object than the magnetic field of the other transmitting coil 125, 130. Based on the influence on the magnetic fields by a metallic object, a non-zero synchronous AC voltage component in particular results in the output signal of the magnetoresistive measuring device. In this case, the measuring apparatus 100 changes the voltages with which the transmitting coils 125, 130 are supplied in the opposite direction until the magnetic fields at the Hall sensors 210, 220 again have the same strength in the respective preferred directions. The voltage present at the connector 155 can be evaluated as a measure of the inequality of the alternating voltages of the transmitting coils 125, 130.

In a second variant, instead of the Hall sensors 210, 220, which determine magnetic fields, sensors are used to determine magnetic field gradients, which are then oriented perpendicular to a magnetic field aligned in the object-free case. In another embodiment, only one such magnetic field gradient sensor is provided.

FIG. 3 shows arrangements of transmitting coils on the measuring apparatus 100 of FIG. 1. For the sake of clarity, pairs of transmitting coils 125/130 lying on top of one other are depicted as circles; magnetic field sensors or magnetic field gradient sensors are not shown. Each of the circles corresponds to one of the arrangements shown in FIG. 4 or FIG. 5. One or a plurality of measuring apparatuses 100 is provided for activating the arrangements 310, with each measuring apparatus 100 being connected to only one of the arrangements 310 during a measurement. A switchover of a plurality of arrangements 310 to one of the measuring apparatuses 100 can occur. Information about the direction, depth, or size of the metallic object to be detected can be determined through appropriate geometric distribution of a plurality of arrangements 310.

It is possible to determine depth with the arrangements in FIG. 3 a or 3 b. It is possible to determine direction or orientation with the arrangements in FIG. 3 c or 3 d. The arrangement in FIG. 3 e can be used for imaging.

FIG. 4 shows arrangements of magnetic field sensors and transmitting coils for the measuring apparatus of FIG. 1. All magnetic field sensors shown in FIG. 4 are oriented vertically in terms of the preferred direction in which they must be exposed to a magnetic field in order to generate a maximum output signal, that is, parallel to a main magnetic field that exists inside the transmitting coils 125, 130. Deviations of the preferred directions of the magnetic field sensors from the specified orientations are innocuous, as long as the deviations are sufficiently small to allow the magnetic field sensors to determine the magnetic field in the orientation used. The transmitting coils 125, 130 are formed as coreless printed coils on opposite sides of a printed circuit board.

In FIG. 4 a, the Hall sensors 210 and 220 of FIG. 2 are respectively arranged centered in one of the transmitting coils 125 or 130. The alignment of the Hall sensors 210 and 220 is antiparallel and their connection is parallel, as shown in FIG. 2.

In FIG. 4 b, the Hall sensors 210 and 220 are oriented in parallel and connected in parallel. The magnetic field generated by the transmitting coils 125, 130 has different orientation inside and outside the transmitting coils 125, 130. The Hall sensors 210, 220 are at a distance from the turns of the transmitting coils 125, 130 such that the magnitudes of the magnetic fields in the object-free case correspond to one another.

In FIG. 4 c, in contrast to the illustration of FIG. 4 b, another, second Hall sensor 220 is provided, with the two second Hall sensors 220 lying opposite one another with respect to the first Hall sensor 210. The alignment of the Hall sensors is parallel, with the distances of the second Hall sensors 220 from the turns of the transmitting coils being chosen such that the total of the magnitudes of the magnetic fields existing at the second Hall sensors 220 in the object-free case corresponds to the magnitude of the magnetic field existing at the first Hall sensor. The Hall sensors 210, 220 are connected in such a way that their output signals are added.

In FIG. 4 d, the principle of the arrangement of FIG. 4 c is extended by providing an additional first Hall sensor 210 in the interior of the first transmitting coil 125. The Hall sensors are connected in such a way that their output signals are added. All Hall sensors are aligned parallel to one another. The distances of the Hall sensors from the turns of the transmitting coil 125 are chosen such that the sum of the magnitudes of the magnetic fields existing at the first Hall sensors 210 corresponds to the sum of the magnitudes of the magnetic fields existing at the second Hall sensors 220.

FIG. 5 shows arrangements of magnetic field gradient sensors and transmitting coils for the measuring device of FIG. 1. In contrast to the illustrations of FIG. 4, the sensors used are magnetic field gradient sensors 510 whose preferred directions run parallel to the surface of the illustrated printed circuit board. In another embodiment, the preferred directions of the magnetic field gradient sensors run parallel to the main field direction. Here as well, deviations of the preferred directions of the magnetic field gradient sensors from the specified orientations are innocuous, as long as the deviations are sufficiently small to allow the magnetic field gradient sensors to determine the magnetic field in the orientation used.

A plurality of magnetic field gradient sensors 510 is respectively connected to one another in such a way that their output signals are added. It is also conceivable that the magnetic field gradient sensors 510 are evaluated separately, for example, in succession. To do this, the outputs of the magnetic field sensors 510 must be connected to the input of the input amplifier 140 by a switch in an alternating manner.

In FIG. 5 a, the transmitting coils 125, 130 are formed on opposite sides of the printed circuit board. The transmitting coils 125, 130 are located next to one another and essentially have a D-shape, with the backs of the D-shapes being parallel and facing one another. The preferred direction of the magnetic field gradient sensor 510 is at an angle of 90° to the direction of the backs of the D-shapes.

In FIG. 5 b, in contrast to the illustration in FIG. 5 a, the magnetic field gradient sensor 510 is arranged on the same side of the printed circuit board as the transmitting coils 125, 130. The backs of the D-shapes surround the place at which the magnetic field gradient sensor 510 is located.

In FIG. 5 c, another magnetic field gradient sensor 510 on the lower side of the printed circuit board is provided in addition to the illustration in FIG. 5 b. The preferred direction of the upper magnetic field gradient sensor 510 is perpendicular to the direction of the backs of the D-shapes, and the preferred direction of the lower magnetic field gradient sensor 510 is perpendicular to the preferred direction of the upper magnetic field gradient sensor 510. In another embodiment, both preferred directions can also be perpendicular to the direction of the backs of the D-shapes.

In FIG. 5 d, a respective arrangement from FIG. 5 b is arranged on the upper side and the lower side of the printed circuit board, with the arrangements at the printed circuit board level being rotated by 90° to one another. The preferred directions of the magnetic field gradient sensors 510 lie perpendicular to one another.

FIG. 6 shows a flow diagram 600 of a method for detecting a metallic object using the measuring apparatus of FIG. 1. In a first step 610, the transmitting coils 125, 130 are supplied with alternating voltages in order to generate oppositely oriented magnetic fields. Next, in a step 620, an output signal, which is dependent on the magnetic field, of a magnetoresistive measuring device in the region of the two magnetic fields is determined. In a step 630, depending on the synchronous AC voltage component of the determined signal, the supply of the transmitting coils with alternating voltages is performed in such a way that the magnitude of the synchronous AC voltage component of the output signal of the measuring device is minimized. Finally, in a step 640, the metallic object is detected if the ratio of the alternating voltages does not correspond to a ratio of the distances of the measuring device from the transmitting coils. 

1. A measuring apparatus for detecting a metallic object, comprising: two transmitting coils configured to generate superimposed magnetic fields; a magnetoresistive measuring device (i) in the region of the two magnetic fields, and (ii) configured to provide an output signal dependent on the magnetic field; and a control device configured to supply the transmitting coils with alternating voltages in such a way that a magnitude of an AC voltage component, which is synchronous with the alternating voltages, of the output signal of the magnetoresistive measuring device is minimized, wherein the control device is configured to detect the metallic object if a ratio of the alternating voltages does not correspond to a ratio of distances of the magnetoresistive measuring device from the transmitting coils.
 2. The measuring apparatus as claimed in claim 1, wherein the alternating voltages are AC voltages that are phase-shifted to one another, in order to change the magnitude and phase of the magnetic fields of the transmitting coils periodically.
 3. The measuring apparatus as claimed in claim 1, further comprising: a plurality of sensors spaced apart from one another for magnetic field determination, wherein the plurality of sensors are aligned with one another and connected to one another in such a way that output signals from the sensors add up to zero when the magnetic fields at the sensors are equal, and wherein main field directions of the transmitting coils and preferred directions of the sensors are essentially parallel to one another.
 4. The measuring apparatus as claimed in claim 3, wherein the transmitting coils lie on top of each other in layers parallel to one another.
 5. The measuring apparatus as claimed in claim 3, wherein one of the sensors of the plurality of sensors is surrounded by one of the transmitting coils and another of the sensors of the plurality of sensors lies outside the transmitting coils.
 6. The measuring apparatus as claimed in claim 1, further comprising: a sensor for determining a magnetic field gradient, wherein main field directions of the transmitting coils run parallel to each other, and wherein a preferred direction of the sensor runs essentially perpendicular or parallel to the main field directions.
 7. The measuring apparatus as claimed in claim 6, wherein: the transmitting coils are arranged essentially next to one another in a layer, and the preferred direction of the sensor runs essentially parallel to this layer.
 8. The measuring apparatus as claimed in claim 6, wherein: the transmitting coils are essentially D-shaped, with the backs of the D-shapes facing one another, and the sensor is arranged between the backs of the D-shapes.
 9. The measuring apparatus as claimed in claim 7, wherein: the sensor is arranged essentially in the layer of the transmitting coils, another sensor is provided in a layer parallel to this layer, and preferred directions of the sensor and the other sensor are perpendicular or parallel to one other.
 10. A method for detecting a metallic object, comprising: supplying two transmitting coils with alternating voltages in order to generate superimposed magnetic fields; determining an output signal, which is dependent on magnetic field, of a magnetoresistive measuring apparatus in the region of the superimposed magnetic fields; wherein the supplying the transmitting coils with alternating voltages takes place in such a way that the magnitude of an AC voltage component, which is synchronous with the alternating voltages, of the output signal of the magnetoresistive measuring device is minimized; and detecting the metallic object if the ratio of the alternating voltages does not correspond to a ratio of the distances of the magnetoresistive measuring device from the transmitting coils.
 11. The method of claim 10, wherein a computer program product includes a program code means configured to perform the method if the computer program product runs on a processing device or is stored on a computer-readable data carrier. 